1. Field of the Invention
The present invention relates to a decoder for convolutional self-orthogonal codes (CSOC's), and more particularly, to a dual threshold decoder wherein a first threshold logic circuit is employed to correct the information bit currently being processed and a second threshold logic circuit is employed in a feedback arrangement to correct selected bits contained in a syndrome register storing a plurality of syndrome bits used to correct the received convolutional self-orthogonal code.
2. Description of the Prior Art
Two systematic procedures are known for threshold decoding of a redundant sequence which includes information signals encoded in accordance with a convolutional, self-orthogonal code. One of these procedures, defined as direct decoding, is embodied in the error-correcting system disclosed in U.S. Pat. No. 3,227,999 issued to D. W. Hagelbarger on Jan. 4, 1966. There, each decoding correction made with respect to an information signal is not fed back to change the nature of a stored syndrome or error pattern representation. If such a change were made, the decoding of subsequently processed information signals would be directly influenced. In other words, in direct decoding there is no feedback connection in the decoding circuitry. Hence, an erroneous decision by the decoder cannot lead to other faulty decisions in subsequent processing. In effect, a positive immunity against error propagation in the decoding process is thereby achieved (but at the expense of reduced error-correcting capabilities).
The other available procedure for processing convolutional self-orthogonal codes is known as feedback decoding. As disclosed in U.S. Pat. No. 3,439,334 issued to J. L. Massey on Apr. 15, 1969, each decoding correction made not only corrects the information bit, but is also fed back to the syndrome register to correct (rightly or wrongly) the affected bits of the syndrome register. It is apparent that in a feedback decoder a bad decoding decision can introduce additional errors in the decoding process.
The problem remaining in the prior art, therefore, is to provide a decoder for convolutional self-orthogonal codes which has greater error-correcting capabilities than a direct decoder but which exhibits considerably less of an error propagation problem than a conventional feedback decoder.